Microelectronic integrated circuits based on patterned semiconductor materials are continuing to evolve towards devices with an extremely high number of circuit elements per unit volume. As the features of these devices are reduced to smaller sizes, the performance of the materials that constitute the device will critically determine their success. One specific area in need of advancement is the electrical insulator used between the wires, metal lines, and other elements of the circuit. As the distances between the circuit elements become smaller, there will be increased problems due to capacitive coupling (crosstalk) and propagation delay, and to increased power dissipation associated with charge depletion from higher capacitance elements. These difficulties can be reduced by preparing the circuit using an insulating material that possesses a dielectric constant (k) as low as possible. It has been conventional to use materials such as silicon dioxide, silicon nitride, and cured silsesquioxanes as insulators. However, the dielectric constants of these materials range from 3.0-7.0 and thus will not be adequate for future circuits.
Fluorinated polymers and fully aliphatic hydrocarbon polymers may have dielectric constants less than about 2.4. However, these low k materials have not been shown to have sufficient thermal and mechanical stability to survive the heat and mechanical stresses occurring during IC fabrication In addition, these polymers typically have chemical properties that are similar in some respects to the chemical properties of photoresist materials commonly used in IC fabrication. Thus, selective chemical removal of photoresist layers deposited on layers of these low k materials might be difficult or impossible.
Numerous photoresist deposition, patterning, development, and selective etching steps could be eliminated from IC fabrication if the low k material used in the IC were itself directly photo-patternable. This can lead to significant manufacturing cost reduction and manufacturing yield enhancement for production. In the Proceedings of the 2000 IEEE International Electron Devices Meeting, pages 253-256, for example, T. Kikkawa reported a process in which a film of methylsilsesquiazane (MSZ) was deposited on a substrate, patterned by exposure to ultraviolet light or to an electron beam, developed, and then thermally cured to form a patterned low k film of methylsilsesquioxane (MSQ).
A problem with Kikkawa's approach, however, is that an MSQ film created in this manner might be weaker than a directly applied and conventionally patterned MSQ film as a result, for example, of component evaporation that weakens the film. An additional problem with this approach is that the photo-patternability function of Kikkawa's film is not separated from the physical properties of the film. That is, the photopatternable feature of the film is part of the structure that provides the mechanical strength and determines the dielectric constant of the film. Consequently, it may be difficult to separately optimize, for example, the photo-patternability, dielectric constant, and strength of the final MSQ film. Another problem with Kikkawa's approach is that the dielectric constant of MSQ is typically in the range of 2.5 to 3.0, which is higher than the values of 1.0 to 2.2 desired by the semiconductor industry.
The coupling of the photo-patternability and physical properties of Kikkawa's film results from the typically homogenous nature of MSZ and MSQ films, which have uniform compositions and structures on length scales greater than molecular length scales (e.g., greater than tens of Angstroms). A general problem with such homogenous media is that their physical properties are often affected when new functions are engineered into the media.
There is a general need for materials in which the physical properties are largely decoupled from desired functions provided by the materials. In particular, there is a clear need for photo-patternable low k materials in which the photo-patternability function may be optimized without substantially affecting bulk physical properties of the resulting patterned material.